Since the 1970s when DuPont processed perfluorinated sulfonic acid resin into perfluorinated sulfonic acid ion exchange membrane and also applied such membrane in chlor-alkali industry and proton exchange membrane fuel cells, perfluorinated sulfonic acid ion exchange resin has been investigated extensively worldwide.
Fluoride-containing ion exchange membrane containing ion exchange groups, especially sulfonic acid groups and carboxylic acid groups, is a more suitable ion exchange membrane which can be used in fuel cells and chlor-alkali electrolytic cells due to its resistance to chemical degradation. U.S. Pat. No. 3,282,875 is the first document disclosed by DuPont on synthesis of sulfonyl fluoride-containing monomer and preparation of sulfonic acid resin, wherein emulsion polymerization in aqueous system was conducted; and functional monomer containing sulfonyl fluoride pendant group has the formula: FO2SCF2CF2OCF(CF3)CF2OCF═CF2 which is utilized widely nowadays. U.S. Pat. No. 3,560,568 is a patent disclosed by DuPont relating to sulfonyl fluoride short pendant group-containing monomer, preparation of sulfonic acid resin and properties thereof, wherein sulfonyl fluoride-containing monomer has the formula: FO2SCF2CF2OCF═CF2. However, procedures of the preparation method are complicated but with low yield. U.S. Pat. No. 3,884,885 and U.S. Pat. No. 3,041,317 described a sulfonic acid resin prepared from perfluorinated sulfonyl fluoride monomers of a new structural formula which is RfCF═CFSO2F. Such monomer has low reaction activity. U.S. Pat. No. 4,358,545 and U.S. Pat. No. 4,417,969 are patents disclosed by The Dow Chemical Company and mainly relate to properties and use of copolymers prepared by copolymerization of short pendant group monomers (CF2═CFOCF2CF2SO2F) and fluorine-containing vinyl monomers such as tetrafluoroethylene. It is very difficult to use the sulfonic acid resin obtained in accordance with this method to prepare thin films by solution coating, and the films obtained does not have sufficient mechanical strength, as U.S. Pat. No. 4,661,411 and U.S. Pat. No. 5,718,647 described. U.S. Pat. No. 4,940,525 discloses a method for preparing copolymer resin from vinylidene fluoride monomer and sulfonyl fluoride short pendant group-containing monomer, wherein said copolymer resin loses perfluorination structure and thereby has poor resistance to corrosion. EP 028969 discloses preparation of a sulfonic acid resin of low EW value, wherein the sulfonyl fluoride monomer used has the monomer structure which is generally adopted nowadays and its EW value is between 575 and 800. EP1451233 reports a method to prepare resin of low EW value by mini-emulsion polymerization. U.S. Pat. No. 7,022,428, U.S. Pat. No. 7,041,409 and U.S. Pat. No. 6,861,489 reports that low-EW value resin is prepared by adopting mini-emulsion polymerization and introducing divinylether-containing monomers during polymerization, wherein the formula of the divinylether-containing monomer is CF2═CF—O—CF2CF2CF2—OCF═CF2 and its EW value is between 625 and 850. GB1034197 discloses perfluorinated sulfonic acid polymer containing sulfonic acid groups and EP1091435 discloses a structure of block sulfonic acid resin, wherein two said polymers are prepared by copolymerization of tetrafluoroethylene monomer and sulfonyl fluoride pendant group-containing vinyl ether monomers (e.g. CF2═CFOCF2CF(CF3)OCF2CF2SO2F), or further introduction of other monomer components containing double bonds but without functional ion exchange pendant groups (e.g. U.S. Pat. No. 4,940,525) into the above-mentioned polymerization system. Polymerization methods include solution polymerization (U.S. Pat. No. 2,393,967, U.S. Pat. No. 3,041,317 and etc.), emulsion polymerization (U.S. Pat. No. 4,789,717, U.S. Pat. No. 4,864,006 and etc.), micro-emulsion polymerization (e.g., U.S. Pat. No. 6,639,011, EP 1172382, U.S. Pat. No. 5,608,022 and etc.), dispersion polymerization, suspension polymerization, mini-emulsion polymerization (EP 1451233) and the like. After obtaining free sulfonic acid group by appropriate hydrolysis of sulfonyl fluoride, these polymers containing sulfonyl fluoride pendant group can function as ion exchange membrane to be applied in the fields of fuel cells, electrolytic cells, diffusion dialysis, catalysis, noble metals recovery and the like.
One of the foremost uses of perfluorinated sulfonic acid resin is to function as membrane material which can be applied in fuel cells. A key requirement for this kind of ion exchange membrane is its ionic conductivity. To increase conductivity, a normal practice known in the art is to increase ion exchange capacity of sulfonic acid resin but mechanical properties decrease as ion exchange capacity increases. The ion exchange resin of high exchange capacity might even be dissolved in water under extreme conditions. As mentioned in EP0031724, ion exchange capacity of the membrane used in electrolytic bath should be between 0.5 and 1.6 mmol/g (dry resin), preferably between 0.8 and 1.2 mmol/g. In the case that the total ion exchange capacity is lower than 0.5 mmol/g, electrical resistance of the membrane would be higher and thereby electrolyzer voltage and energy consumption are higher too, which does not satisfy industrial application. In the case that the total ion exchange capacity is higher than 1.6 mmol/g, the membrane materials have poor mechanical properties and thereby the life span and utilization will be limited. To increase exchange capacity and decrease loss of mechanical properties to the greatest extent, some alternative methods are to utilize composite membranes. For example in U.S. Pat. No. 5,654,109 and U.S. Pat. No. 5,246,792, bilayer or three-layer membrane materials were composited, wherein the inner membrane of high EW value undertakes the responsibility for conferring mechanical strength while the outer membrane(s) of low EW value takes responsibility for ion conduction. Multiple-layer membranes of different ion exchange capacity were combined in U.S. Pat. No. 5,981,097; while the composite membrane was obtained by combining the biaxial stretched polytetrafluroethylene porous membrane and resin of low EW value in U.S. Pat. No. 5,082,472. Although the above-mentioned methods retain mechanical properties of membranes to some extent, those methods are relatively poor at improving uniformity and conductivity of ion conduction.
In order to enhance mechanical strength and size stability of ion exchange membrane, one solution is to modify resin structure through a method well known in the art that crosslinkable groups are introduced into resin structure. For example, as used in US 20020014405 and U.S. Pat. No. 6,767,977, diene monomers were introduced into resin structure. CN 200480033602.1 discloses a method for introducing nitrile groups into polymerization system, wherein the nitrile groups were crosslinked after treatment and thereby mechanical strength of the membrane was enhanced. CN 200480033631.8 discloses a method for introducing bromine, chlorine or iodine groups into polymerization system followed by crosslinking in the presence of electron beam. An alternative solution is to shorten sulfonyl fluoride pendant group of comonomer to thereby enhance mechanical strength of membrane materials at the same time of increasing ion exchange capacity. However, as mentioned in U.S. Pat. No. 6,680,346, the polymers synthesized from short pendant group sulfonyl fluoride-containing monomers are subjected to cyclization due to different polymerization conditions, which results in chain transfer during polymerization and thereby causes a decrease in molecular weight and mechanical strength of the materials. As the molar ratio of short pendant group sulfonyl-containing monomer to tetrafluoroethylene monomer increases, said side reaction may be further promoted, which limits increase of ion exchange capacity and material stability.
When applying perfluorinated sulfonic acid resin to fuel cells as membrane materials, which is the most important application of perfluorinated sulfonic acid resin, key requirements for this kind of membrane electrode are its chemical stability and capability of enhancing electrode catalyst's resistance to carbon monoxide poisoning, wherein said membrane electrode is formed from ion exchange membrane and catalyst layer. The membrane electrodes of fuel cells that are extensively investigated and exemplified nowadays generally have the working temperature between 25° C. and 80° C. Catalyst layer of membrane electrode may be subjected to an outbreak of poisoning once the CO content in the circumstance reaches 10 ppm. To overcome many difficulties of membrane electrodes of low-temperature fuel cells that can hardly be resolved, for example, how to increase activity and utilization of catalyst, how to enhance electrode catalyst's resistance to carbon monoxide poisoning, and the like, an effective resolution is to increase operating temperature of fuel cells. Resistance of membrane electrode catalyst to CO will be increased to about 1000 ppm when the temperature exceeds 100° C. Development of high-temperature proton exchange membrane may better improve electrical efficiency of fuel cells and reduce costs of cell system so as to better satisfy commercialization of fuel cells. At present, main countries in the world researching fuel cells start to put massive manpower and material resources in the research. Current sulfonic acid resin comprising long pendant groups cannot meet requirements for use at high temperature.